Re: [ESPResSo-users] dielectric interfaces simulation

[Stefan Kesselheim]

Hi,

On Feb 6, 2014, at 9:07 PM, Xikai Jiang <address@hidden> wrote:

> iccp3m $Nstart eps_out 1 max_iterations 60 convergence 1e-3 relax 0.7 areas 
> $areas normals $normals epsilons $epsilons ext_field $ext_fields
> 
> In the iccp3m.c code, my understanding is that the values of ext_field are 
> applied to the wall atoms only. In the last email, you mentioned that I 
> cannot apply this field to the induced charges. Do you mean that I can't 
> apply the field to wall atoms? If so, do I need to apply the field to the 
> charges between two plates?
> 
> One more question, does Espresso 3.2.0 supports reading in external 
> potentials, which are then applied to charged particles weighted with the 
> according charge? (Section 4 in the paper you suggested 
> http://www.mdpi.com/1099-4300/15/11/4569)
> Since I haven't figure out how to do that yet.

OK - There is two things, that are very similar but quite the opposite of one 
another :-).

Point one: If you want to apply a homogenous external field to some object, 
lets say a dielectric or conducting sphere, and want to calculate the charges 
that are induced on the surface within Espresso, you use the iccp3m .. 
ext_field. The idea is that this includes a homogenous external field in the 
calculations. It is clear how this should work for objects which to not cross 
periodic boundary conditions. Probably it also gives something meaningful if 
you don't go to the limit epsilon_wall->infinity. But if you do, I'm not 
exactly sure what happens. But it should be clear that the voltage then is 
E*L_Box and not E*L_Channel. This you can make clear to yourself when 
considering what you'd expect for two different dielectrics …

Point two. If you have some external solution for the electric field from - 
let's say - a finite element solution of the Poisson equation, the induced 
charges of this external potential are - in a matter of speaking - already in 
your solution. During the MD, you only need to calculate the additional induced 
charges, which make sure you get the right field caused by the ions. Therefore 
the external potential is not applied to the particles.

Answer to the last question: No, Espresso 3.2.0 does not support that. I have 
code, which does exactly that, which was made exactly for that workflow. We 
wanted to apply it but so far it has not been used in production. I still have 
to merge it into the current Espresso master.

I hope that clarified a few things. 
Cheers
Stefan